Calcium sulphate anhydrite mortar

ABSTRACT

Particulate anhydrite, of whose particles from about 2 to 6% by weight have a particle size greater than about 5 mm, from about 13 to 23% by weight have a particle size of from about 5 to 3.15 mm, from about 33 to 40% by weight have a particle size of from about 3.15 to 1 mm, from about 17 to 23% by weight have a particle size of from about 1 to 0.2 mm, and from about 17 to 33% by weight have a particle size less than about 0.2 mm, will provide a mortar developing high early strength without requiring an excessive amount of water. If the particles having a size less than about 0.2 mm are activatingly ground to increase their energy content to above the minimum possible under ambient pressure and temperature conditions, a particularly speedy initial rise in strength takes place.

This is a divisional, of application Ser. No. 319,393, filed Dec. 29,1972.

The present invention relates to a particulate anhydrite (CaSO₄) whoseparticle size is so chosen as to make it especially effective for use asa high strength mortar. Anhydrite, which is anhydrous calcium sulphate,is converted by water into gypsum (CaSO₄, 2H₂ O) during setting. Oncethe mortar mass has set, it is bound together with 3-dimensionalstability by interlocked crystals of gypsum. This property makesanhydrite particularly suitable for use as a mortar in the presentinvention. By the use of certain accelerators of set, generallysolutions of various salts especially sulphates, mortars have beenproduced for various purposes.

The strength of this gypsum-bound mass will vary with time as settingproceeds but, at any given time, it will depend on:

A. THE DEGREE OF CONVERSION OF ANHYDRITE TO GYPSUM;

B. THE RESIDUAL MOISTURE CONTENT;

C. ITS INTERNAL STRUCTURE; AND

D. THE FIRMNESS OF ATTACHMENT OF THE BINDING PHASE (GYPSUM) TO THEINTERCALATED PHASE (FILLER).

In order to achieve a high degree of conversion to the dihydrate(gypsum), the anhydrite is normally finely ground (e.g. the proportionof the anhydrite having a particle size greater than 0.1 mm issubstantially zero; Blaine values are over 4,000 cm² /g) and the mosteffective possible accelerators of set are added to the anhydrite.

Since the more finely ground the anhydrite, the larger the quantity ofmixing water required, it would be better to keep the proportion offinely ground material to a minimum, in order to keep as low as possibleboth the residual water content and the internal heating when a largequantity of anhydrite mortar is used. The intercalated phase ispreferably anhydrite having a specific particle size distribution. Ithas been shown that the adhesion between the gypsum binding phase andthe intercalated phase is better when the intercalated phase consists ofanhydrite rather than of other substances, due to the chemical affinityof gypsum for anhydrite.

We have surprisingly found that the opposing requirements for thequantity of mixing water, the rate of hydration, the rise in temperaturefollowing gypsum formation, the internal structure, the residual watercontent and the increase in strength may be met by a particulateanhydrite, of whose particles from 2 to 6% by weight have a particlesize greater than 5 mm, from 13 to 23 % by weight have a particle sizeof from 5 to 3.15 mm, from 33 to 40 % by weight have a particle size offrom 3.15 to 1 mm, from 17 to 23 % by weight have a particle size offrom 1 to 0.2 mm and from 17 to 33 % by weight have a particle size lessthan 0.2 mm. The particles having a size less than 0.2 mm are preferablyactivatingly ground, since this will give an accelerated increase instrength performance.

The important fraction of the particulate anhydrite in the achievementof high early strength is that fraction having a particle size less than0.2 mm. We have found that the initial rise in strength takes place morequickly if the fraction having a particle size less than 0.2 mm isactivatingly ground in what are termed "disintegrators." After theanhydrite raw material has been crushed, the fine material is separatedeither completely or partially, is activatingly ground and is then addedto the coarse material. Alternatively, the anhydrite raw material isfirst crushed so that the proportion of particles having a size lessthan 0.2 mm is small and then separately ground active fine anhydriteparticles are added in the required amount.

The effect of the activating grinding is to increase the energycontained in the fine particles of anhydrite without increasing theirtemperature. If a solid body has a higher energy content than theminimum possible under the relevant pressure and temperature conditions,it is said to be in an "active state." Certain grinding processes arecapable of causing the material to assume an active state: suchprocesses are referred to herein as "activating grinding" processes andthe term "activatingly ground" is to be construed accordingly.

In such activating grinding processes, the whole of the mechanicalenergy expended in grinding is not converted into heat; a smallproportion is absorbed by the particles of solid material being groundto raise those particles to a higher energy state. This energy isgenerally absorbed by means of changes in the lattice structure or bythe creation of lattice defects in the solid material. The process ofactivating grinding and the theory on which the process is based arediscussed in greater detail in the articles "Mechanophysikalische undmechanochemische Reaktionen beim Schwingmahlverfahren" (Mechano-physicaland mechano-chemical reactions in the oscillatory grinding method) by E.Gock and M. Korn in the Anlagen-Technik, October 1971, pages 77 to 80and "Zur tribomechanischen Aktivierung von Festkorpern unter Anwendunghoher mechanischer Energien" (Tribomechanical activation of solid bodieswhen using high mechanical energies) by Johannes Hint in Silikattechnik21 (1970) Part 4, pages 116 to 121. We particularly prefer to use the"disintegrator" described in the article "Zur tribomechanischenAktivierung von Festkorpern unter Anwendung hoher mechanischerEnergien," that is a mill in which the grinding is effected by aplurality of pins mounted on a rotor; the repeated shocks experienced bythe particles as they are struck by the pins causes a substantialincrease in the energy content of the particles.

We prefer that the water : anhydrite weight ratio in the mortar shouldbe from 0.09:1 to 0.12:1; with such a ratio, this anhydrite binderprovides the mortar with 24 hour compression strengths greater than 100kp/cm², even when the mortar is used for filling large cavities, forbuilding large scale walls or for building dams, and even undertemperature and atmospheric moisture conditions which are markedlydifferent from those of a normal climate. This anhydrite binder is thusespecially suitable for use in mining operations. This following Tablecompares the increase in strength of two types of anhydrite mortars, oneusing a ground anhydrite having a conventional range of particle sizes,and the other using anhydrite according to the present invention. Eachanhydrite has the same percentage space filling. The compressionstrengths were determined using prisms of mortar of size 4 × 4 × 6 cm,tested in a moist condition.

The particle size distribution of conventionally used anhydrite normallyis as follows:

    >0.2 mm               0.3%                                                     0.09 - 0.2 mm        0.8%                                                     0.06 - 0.09 mm       4.0%                                                    <0.06 mm              94.9%                                               

The distribution of particle sizes of the anhydrite of the presentinvention was as follows:

    >5.0           mm       2.5 %                                                 5.00 - 3.5     mm       19.4 %                                                3.15 - 1.00    mm       35.3 %                                                1.00 - 0.2     mm       18.4 %                                                0.2 - 0.09     mm       3.8 - 4.1 %                                           0.09 - 0.063   mm       6.8 - 7.2 %                                           <0.063         mm       13.5 - 14.2 %                                     

One anhydrite used in the present invention was activatingly ground in a"disintegrator," whereas the other anhydrite was ground underconventional conditions. In both types of mortar, the ratio of water toanhydrite was 0.09 : 1, the water being supplied by a solution ofsulphate accelerators containing 3.5 % by weight potassium sulphate and6.5 % by weight ferrous sulphate.

                  TABLE                                                           ______________________________________                                        Setting time (hours)                                                                        Compression strengths (kp/cm.sup.2)                             at 20°C/95% relative                                                                 of anhydrite mortars                                            humidity                                                                      ______________________________________                                                         Invention                                                                  Conven-  non activating                                                                            activating                                               tionally grinding    grinding                                                 used                                                            24             85      120         168                                        72            100      224         246                                        ______________________________________                                    

Mortar comprising the anhydrite of the invention may be used to formbulkheads to support the roof at or adjacent the face of a mining tunnelor shaft. These bulkheads are commonly made from waste materials orwood, but bulkheads made from either of these known materials tend to bedifficult to install and will sink before they assume any pressure. Theloss of height tends to be some 40 to 50 % of the average thickness.Furthermore, bulkheads of this type tend to leak, which result in lossesdue to weathering and in the danger of fire in abandoned workings. Wehave now discovered that suitable bulkheads may be manufactured from amortar containing the anhydrite of the invention. The advantages of thisare considerable. Thus, the anhydrite mortar can be supplied to therequired position through pipes and may be blown right up to directlyunder the roof. The mortar achieves a high early strength and thus willbear the weight of the roof almost immediately. Since the conversion ofanhydrite to gypsum involves a volume increase, the setting bulkheadwill tend to grow towards the sinking roof.

Moreover, the bulkhead will be continuous, without any gaps or aperturesand will lie completely up against the bedrock. The bulkhead isimpermeable to air and other gases and thus avoids losses due toweathering and minimizes the risk of fires in abandoned workings.

Of course, it is normally necessary to add an accelerator of set to themortar in order to induce the anhydrite to react quickly with the water.Such an accelerator may be any one of those conventionally used, but weparticularly prefer to use a water-soluble sulphate. Ferrous sulphateand potassium sulphate are particularly preferred and we most prefer touse a mixture of ferrous sulphate and potassium sulphate. The sulphateaccelerator is preferably used in an amount of about 1 % by weight,based on the weight of the anhydrite. If a mixture of ferrous sulphateand potassium sulphate is used, the relative amounts preferably rangefrom a weight ratio of 2 : 8 to 5 : 5 (potassium sulphate : ferroussulphate), most preferably about 3.5 : 6.5. Since the sulphateaccelerator is generally used in the form of an aqueous solutioncontaining about 10 % by weight of sulphate accelerator, sufficientsolution to supply about 1 % of accelerator, based on the weight ofanhydrite, will normally supply the water needed for the mortar.

We claim:
 1. A mortar consisting essentially of particulate anhydriteand water, the weight ratio of water to anhydrite being from about 0.09: 1 to about 0.12 : 1, wherein from about 2 to 6% by weight of theparticles of particulate anhydrite have a particle size greater thanabout 5 mm, from about 13 to 23% by weight have a particle size of fromabout 5 to 3.15 mm, from about 33 to 40% by weight have a particle sizeof from about 3.15 to 1 mm, from about 17 to 23% by weight have aparticle size of from about 1 to 0.2 mm, and from about 17 to 33% byweight have a particle size less than about 0.2 mm.
 2. A mortar asclaimed in claim 1, wherein the particles having a size less than 0.2 mmhave an energy content greater than the minimum possible under ambientpressure and temperature conditions.
 3. A mortar as claimed in claim 1,additionally including an accelerator of set selected from the groupconsisting of ferrous sulphate, potassium sulphate and mixtures offerrous sulphate and potassium sulphate.
 4. A mortar as claimed in claim2, additionally including an accelerator of set selected from the groupconsisting of ferrous sulphate, potassium sulphate, and mixtures offerrous sulphate and potassium sulphate.
 5. A mortar as claimed in claim1, additionally including a mixture of potassium sulphate and ferroussulphate in which the weight ratio potassium sulphate : ferrous sulphateis from about 2 : 8 to about 5 :
 5. 6. A mortar as claimed in claim 2,additionally including a mixture of potassium sulphate and ferroussulphate in which the weight ratio potassium sulphate : ferrous sulphateis from about 2 : 8 to about 5 :
 5. 7. A mortar as claimed in claim 2,additionally including a water-soluble sulphate in an amount of about 1%by weight, based on the weight of the anhydrite.